1. Technical Field
The present invention relates to a switched reluctance motor (SRM), and more particularly to a SRM driving circuit.
2. Prior Art
FIG. 1 shows a construction of a stator and a rotor of a general SRM, in which coils 4,5,6 are wound on poles 1,2,3 of the stator, and if the magnetic flux is generated by applying phase excitation signals to the coils 4,5,6, the rotor 7 is rotated.
FIGS. 2A to 2E show several conventional 3-phase SRM driving circuits, including an R-dump circuit shown in FIG. 2A, a q+1 circuit in FIG. 2B, a C-dump circuit in FIG. 2C, a nonsymmetrical bridge circuit in FIG. 2D, and a bifilar winding circuit in FIG. 2E. In the conventional 3-phase SRM driving circuits, if phase excitation signal is applied to the coils 4,5,6 with a predetermined phase difference, the SRM is driven, and the magnetic energy of the coils 4,5,6 is returned to the main power source. Hereinafter, the operation of the conventional SRM driving circuit will be described, mainly referring to the R-dump circuit in FIG. 2A.
The conventional R-dump circuit comprises coils 4,5,6 interconnected in parallel, switching sections T.sub.1, T.sub.2, T.sub.3 for switching the excitation current passed through the coils 4,5,6 by controlling the phase excitation signal, diodes D.sub.1,D.sub.2,D.sub.3 respectively connected to the coils 4,5,6, resistors R.sub.1,R.sub.2,R.sub.3 respectively connected to the diodes D.sub.1,D.sub.2,D.sub.3, and a condenser C.sub.1 for accumulating the current passing through the resistors R.sub.1,R.sub.2,R.sub.3, in which if the magnetic flux is generated by controlling the phase excitation signal, the rotor 7 of the SRM is rotated, and the operation of which will be described hereinafter in detail.
First, if the main power is applied, a first phase excitation signal Sa is applied to the transistor of the switching section T.sub.1, by which the transistor is turned on, and then a current passes through the coil 4 and the magnetic flux is generated.
After the above process, if the switching section T.sub.1 is turned off by stopping to supply the first phase excitation signal Sa and the switching section T.sub.2 is turned on by applying a second phase excitation signal Sb to the switching section T.sub.2, the excitation current, which was, stared at the coil 4 as magnetic energy, flows through the diode D.sub.1 and the resistor R.sub.1 to the condenser C.sub.1 to be stored therein as electric energy, and a current passes through the coil 5, so that magnetic flux is generated.
Further, if the switching section T.sub.2 is turned off by stopping to supply the second phase excitation signal Sb and the switching section T.sub.3 is turned on by applying a third phase excitation signal Sc to the switching section T.sub.3, the excitation current stored at the coil 5 as magnetic energy flows through the diode D.sub.2 and the resistor R.sub.2 to the condenser C.sub.1 to be stored therein as electric energy, and a current passes through the coil 6, so that the magnetic flux is generated. As is known by the above description, in the conventional SRM driving circuit, the magnetic energy stored at the coils 4,5,6 is stored at the condenser C.sub.1 as electric energy by performing the above-described operations continuously and repeatedly.
Meanwhile, in the q+1 circuit shown in FIG. 2B, the resistors R.sub.1, R.sub.2, R.sub.3 in the R-dump circuit are absent and a switching section T.sub.4, which is for chopping, is connected between the main power Vdc and the coils 4,5,6. In the C-dump circuit shown in FIG. 2C, the phase excitation current of the R-dump circuit in FIG. 2A is first stored at a condenser Cd as electric energy, and then the electric energy can be stored at the condenser C.sub.1 through a coil Ld by switching of the switching section Ts. In the nonsymmetrical bridge circuit in FIG. 2D, switching sections T.sub.4,T.sub.5,T.sub.8 are respectively connected between the main power Vdc and the coils 4,5,6, and the phase excitation current is stored at the condenser C.sub.1 as electric energy through the diodes D.sub.1,D.sub.2,D.sub.3. In the bifilar winding circuit in FIG. 2E, the coils 4,5,6 induce the phase excitation current by inductive coupling circuits L.sub.1,L.sub.2,L.sub.3, and the phase excitation current induced is returned to the condenser C.sub.1 through diodes D.sub.4,D.sub.5,D.sub.6, which form the discharging path of the phase excitation current.
However, the loss of energy is too large in the R-dump circuit, the space efficiency of the switching section T.sub.4 is low and the high speed operation is restricted due to the mutual inductance in the q+1 circuit, and the high speed operation of the C-dump circuit is disadvantageous.
Further, the manufacturing cost of the nonsymmentrical bridge circuit is very expensive, and the volume of the motor of the bifilar winding circuit is too large and its manufacture is difficult.
Generally, in case there is no phase difference in an SRM, in other words, when the poles 1,2,3 of the stator coincide with the protrusions of the rotor 7, the inductance of the coils 4,5,6 are maximized, while in case the phase difference between the poles 1,2,3 of the stator and the protrusions of the rotor 7 is 45.degree., the inductance of the coils is minimized.
In a general SRM, the excitation is initiated when the phase difference is 45.degree., that is when the inductance of the coils is starting to increase. If the excitation is initiated when the inductance of the coils decreases, the motor is braked.
FIG. 2F and FIG. 2G show two conventional 4-phase SRM driving circuits. In the 4-phase SRM driving circuit in FIG. 2F, pairs of N-MOS transistors (M.sub.1,M.sub.2), (M.sub.3,M.sub.4), (M.sub.5,M.sub.6), (M.sub.7,M.sub.8) are respectively interconnected in series, the coils 4,5,6,8 are respectively connected between the sources of the first N-MOS transistors M.sub.1,M.sub.3,M.sub.5,M.sub.7 and the drains of the second N-MOS transistors M.sub.2,M.sub.4,M.sub.6,M.sub.8, the cathodes of first diodes D.sub.8,D.sub.10,D.sub.12,D.sub.14 are connected to the sources of the first N-MOS transistors M.sub.1,M.sub.3,M.sub.5,M.sub.7, the anodes of second diodes D.sub.9,D.sub.11,D.sub.13,D.sub.15 are connected to the drains of the second N-MOS transistors M.sub.2,M.sub.4,M.sub.6,M.sub.8 and the cathodes of the second diodes are connected to the power source Vdc, and the drains of the first N-MOS transistors M.sub.1,M.sub.3,M.sub.5,M.sub.7 are also connected to the power source Vdc.
If a pulse width modulation (PWM) signal of high level is applied to the gates of a pair of N-MOS transistors M.sub.1,M.sub.2, the N-MOS transistors M.sub.1,M.sub.2 are turned on and a current flows through the coil 4.
If a pulse width modulation signal of low level is applied to the gates of N-MOS transistors M.sub.1,M.sub.2 after a predetermined time passed, the N-MOS transistors M.sub.1,M.sub.2 are turned off and a current discharging path comprised of the first diode D.sub.1, the coil 4 and the second diode D.sub.9 is formed.
Then, the current stored as magnetic energy at the coil 4 is starting to flow through the current discharging path, so that it decreases gradually. Therefore, the magnetic energy is stored through the current discharging path at the capacitor C.sub.1, which is connected between the positive terminal and the negative terminal of the power source Vdc, as electric energy.
Further, when the inverse-phase braking is performed, more current than the applied current is returned from the coil 4 through the first and the second diodes D.sub.8,D.sub.9 to the capacitor C.sub.1, and thereby the voltage is elevated. Therefore, to prevent this, the resistor R4 in series with the N-MOS transistor M.sub.9 is connected between the positive terminal and the negative terminal of the power source Vdc in parallel with the capacitor C.sub.1.
If large voltage is applied to the capacitor C.sub.1, a signal of high level is applied to the gate of the N-MOS transistor M.sub.9 so that voltage is applied to the resistor R4.
FIG. 2H shows wave forms at several sections of the circuits in FIG. 2F, in which a shows the change of the inductance of the coil 4 according to the phase Q, b shows the change of the phase current flowing through the coil 4, c shows the wave form of the phase excitation signal applied to a pair of the N-MOS transistors M.sub.1,M.sub.2, and d shows the wave form of torque.
FIG 2G shows another conventional 4-phase SRM driving circuit, in which the first diodes D.sub.8,D.sub.10,D.sub.12,D.sub.14 and the first N-MOS transistors M.sub.1,M.sub.3,M.sub.5,M.sub.7 of the circuit in FIG. 2F are absent.
FIG. 2I shows wave forms at several sections of the circuits in FIG. 2G, in which a shows the change of the inductance of the coil 4 according to the phase Q, b shows the change of the phase current flowing through the coil 4, c shows the wave form of the phase excitation signal applied to the N-MOS transistor M2, and d shows the wave form of torque.
In FIG. 2G and FIG. 2I, if a phase excitation signal of high level as shown in FIG. 2I a is applied to the gate of the N-MOS transistor M.sub.2, the N-MOS transistor M.sub.2 is turned on, thereby current is starting to flow through the coil 4 and its flow is gradually increased while the phase excitation signal is in high level.
If a phase excitation signal of low level is applied to the gate of the N-MOS transistor M.sub.2 while the current increases, the N-MOS transistor M.sub.2 is turned off and the current accumulated at the coil 4 as magnetic energy circulates in a closed loop through the diode D.sub.9. Therefore, the current flowing through the coil 4 is changed as shown in FIG. 2I b according to the switching state of the N-MOS transistor M.sub.2.
However, when the N-MOS transistor M.sub.2 is in the state of turned-off, because the current saved as magnetic energy at the coil 4 circulates, the current is not decreased quickly in the closed loop comprising the coil 4 and the diode D.sub.9, and thereby, because fairly large quantity of current lasts to circulate in the loop even while the inductance decreases, the SRM is braked and torque as shown in FIG. 2 d is applied to the SRM.
That is, the circuit of FIG. 2F has good driving efficiency, but it requires an overvoltage protective circuit because the voltage of the capacitor is increased when the motor is braked, while the circuit of FIG. 2G does not elevate the voltage of the capacitor when the motor is braked, but its driving efficiency and velocity are low.